Conventionally, a friction clutch and a brake are provided in a vehicle such as a four-wheel car or a two-wheel vehicle. The friction clutch is a mechanical device which transfers rotational drive force of a prime mover such as an engine to a driven body such as wheels, or shuts off the transfer of the rotational drive force. The brake is a mechanical device which brakes rotational motion against the rotational drive force of the prime mover. In general, such a friction clutch and such a brake include two plates which are disposed in oil to face each other. The two plates are a wet friction plate which is formed by attaching frictional sheets on the surface of a flat annular metal core, and a plain plate which has no frictional sheet. Transfer of the rotational drive force or braking is effected by pressing the wet friction plate and the plain plate against each other.
Such a friction clutch and such a brake are always required to reduce so-called drag torque in order to improve the fuel consumption of a vehicle in which the friction clutch or the brake is installed. Drag torque is torque which is transferred between the wet friction plate and the plain plate separated from each other, due to viscous resistance of the oil present between the two plates, when a difference in rotational speed is present between the two plates. Drag torque is one cause of an increase in the fuel consumption of a vehicle.
Therefore, a clutch friction plate (wet friction plate) which reduces drag torque is proposed as disclosed in, for example, Patent Document 1. The proposed clutch friction plate has small frictional sheets which are arranged in the circumferential direction thereof such that a gap serving as an oil groove is provided between adjacent frictional sheets. The clutch friction plate also has cutout spaces which are wider than the oil groove and which are provided at intervals in the row of the frictional sheets. Thus, the clutch friction plate has an enhanced performance of draining clutch oil, to thereby reduce drag torque. In the case of the clutch friction plate disclosed in Patent Document 1, whereas the reduction ratio of the total area of the frictional sheets is 16.6%, the reduction ratio of drag torque is 24.2%.